A switching power conversion circuit principally comprises a switching circuit and at least a magnetic element. The magnetic element is for example a transformer or an inductor. By alternately conducting or shutting off the switching circuit of the switching power conversion circuit, the input power is converted by the magnetic element into an output voltage including at least a regulated voltage level. The output voltage is used to drive the load that is connected to the output terminal of the switching power conversion circuit. In addition, the magnetic element can be used to achieve voltage-increasing, voltage-decreasing, filtering or other purposes.
When the switching circuit of the switching power conversion circuit is alternately conducted or shut off, the magnetic element will generate corresponding magnetic flux according to the formula B=V×Ton/(Ts×A), in which B is the magnetic flux, V is the input voltage of the switching power conversion circuit, Ton is the on duration of the switching circuit, Ts is the coil turn of the magnetic element, and A is an effective area passing through the magnetic flux. Generally, the on duration Ton of the switching circuit is dependent on the output voltage to be transmitted to the load. As the output voltage is increased, the on duration Ton of the switching circuit is extended. Since the coil turn Ts and the effective area A are determined after the switching power conversion circuit is fabricated, the magnetic flux B is changed with the input voltage V and the on duration Ton of the switching circuit.
The switching circuit is usually operated at a constant switching frequency. As the output voltage is increased, the on duration Ton of the switching circuit is extended. The magnetic flux B generated may exceed the maximum saturation magnetic flux of the magnetic element. If the magnetic element is saturated, the impedance of the magnetic element becomes very small. Under this circumstance, the current flowing through the magnetic element becomes extremely large. Consequently, the magnetic element or other components of the switching power conversion circuit may be immediately burned out.
Moreover, the conventional switching power conversion circuit has a bootstrap circuit for driving the switching circuit at the high-voltage terminal of the switching power conversion circuit. Since the bootstrap circuit is very costly, the use of the bootstrap circuit increases extra cost. For solving these problems, some literatures have disclosed a method of adjusting the locations or the number of the switching circuit. Such a method, however, fails to precisely control the output current or the output voltage of the switching power conversion circuit, and thus the operation of the load is not optimized.
Therefore, there is a need of providing a switching power conversion circuit to obviate the drawbacks encountered from the prior art.